spawn.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'appscript' # gem install rb-appscript

class Tab
  include Appscript

  def initialize(directory)
    app("System Events").application_processes["Terminal.app"].keystroke("t", :using => :command_down)
    @terminal = app("Terminal")
    @tab = @terminal.windows.first.get.tabs.last.get
    @terminal.do_script("cd #{directory}", :in => @tab)
  end
end

ARGV[0].to_i.times do
  Tab.new(ARGV[1] || Dir.pwd)
end

Usage

$ spawn 3 
# => open 5 new Terminal tabs, cd to the current directory

$ spawn 5 ~/Code/project-x
# => Open 5 new Terminal tabs, cd to ~/Code/project-x